The proposed scoping review is part of an ongoing study of blood DNA hydroxymethylation among twins discordant for incident type 2 diabetes (iT2DM-dTwin) in the National Heart, Lung, and Blood Institute (NHLBI) Twin Study (NTS) (NTS iT2D-dTwin study) aiming to address part of specific aim 3 in an NHLBI R15-funded study of hydroxymethylation, incident type 2 diabetes and incident obesity (R15HL152330) concerning type 2 diabetes in the USA.